Increasing interest in developing lubricants that provide low friction and which are energy efficient in equipment for both the automotive and industrial lubricants industries is a macro trend across the lubricant industry today. One important way to reduce friction is to use friction-modifier additives in lubricant compositions. There has been considerable research carried out to explore the performance and mechanisms of action of friction-modifying additives across the lubricant industry. During this time, tribological tools for studying surfaces have greatly enhanced our understanding of how additives perform. Researchers have shown that friction can be reduced in boundary lubricating conditions by adsorbing or reacting additives on metal surfaces to form thin low-shear-strength films. Furthermore some of these additives can have a significant effect on friction in the mixed lubrication regime.
Two examples of friction-modifying additive chemistries are organic friction modifiers and organo-molybdenum compounds. The former are usually long-chain polar compounds based on carboxylic acid, amine, ester and alcohol groups. Examples include glycerol mono-oleate, oleylamide, stearic acid and trimethyolpropane esters. These tend to function through their polar heads absorbing on to surfaces with the lipophilic tail aligned perpendicular to the surface. There are some practical challenge in using these materials in lubricant compositions. For example esters are prone to hydrolysis if there is ingress of water into the lubricant. Amine containing materials are known to cause elastomer incompatibility issues. Amides, such as oleylamide, are known to have a high degree of surfactancy character and can lead to emulsion formation. Carboxylic acids can react with metals to form carboxylate salts that are sometimes not desired.
When formulating lubricants, it is highly desirable that all additives, including friction-modifiers, be soluble in the composition. Such solubility is preferably maintained across a wide range of temperature and other conditions in order to enable shipping, storage, and/or prolonged use of these compositions.
Lubricant additives that provide significant friction modification benefits without the disadvantages of current additives, such as hydrolytic instability, and that are also readily soluble in the lubricant base oil would be highly beneficial to the lubricant industry.